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combination and decomposition reactions (Combination reactions (synthesis…
combination and decomposition reactions
Combination reactions
(synthesis reactions):
2 or more substances chemically bonded together = produce a single compound
simple
chemical substances are joined together to make 1 more
complicated
chemical substance
the reactions can occur between 2 elements, or compounds
(many combination reactions require the reactants to be heated, in order to make the reaction occur)
- combination reactions are exothermic
2 types of reactions:
Ionic Reactions
:
reactions between
metal & non-metal
elements
(Ionic bonding / forms ionic compounds)
metal atoms loose electrons
to form positive ions (anions)
non-metal atoms gain electrons
to form negative ion (cations)
ions formed now have full (stable) outer shells
electrons are transferred
( from metals to non-metal)
oppositely charged ions attract
each other so the
bonds
holding the ionic compound together are
strong
(hard to separate =
not very reactive
)
the ionic product formed is
usually a solid
e.g. 2Mg(s) + O2(g) = 2MgO
(s
)
Molecular / Covalent Reactions
:
reactions between
non-metal & non-metal
elements
(molecular bonding / forms molecular compounds)
bothe tyes of atoms (non-metals)
need more electrons
to gain full (stable) valence shells
electons are shares
(between atoms)
bonds
between the molecules are
weak
(so easily separated =
more reactive
)
molecular compounds may be
liquid or gas
e.g.
S(s) + O2(g) = SO2
(g)
some non- metals
can't exist on their own:
O2
H2
Cl2
I2
N2
(
S & C
can exist on their own)
Decomposition reactions:
A decomposition reaction is a type of chemical reaction in witch a single compound breaks down into 2 or more simpler elements &/or compounds.
1 complex compound = 2 or more simpler elements &/or compounds
reactions often involve an energy source such as heat, light or electricity
(
but most reactions will occur slowly on there own in normal conditions)
decomposition reactions are exothermic
2 types of decomposition:
Thermal Decomposition:
decomposition reactions
speed up by heating
require the
reactant substances to be heated
,
in order for the decomposition to occur
3 types
of compounds that undergo
decomposition by heating
carbonates
hydrogen carbonates
hydroxides**
Carbonates & hydroxides of group 1 metals,
don't decompose by heat
(thermal decomposition)
(all hydrogen carbonates decompose on heating)
3 types
Decomposition of
Metal Carbonates
by heat
metal carbonates, (that decompose) leave a
residue of metal oxide & evolve a CO2 gas
(the gas can be tested with the lime water test)
metal carbonate (heat) = metal oxide + carbon dioxide
Decomposition of
Metal Hydroxides
using heat:
leaves a
residue of metal oxide & evolves H2O
(the water will condense on the test tube)
Metal hydroxide (heat) = metal oxide + water
(some metal hydroxides decompose so easily that they can't be kept solid (easily) because they just decompose back into liquids)
Decomposition
Metal Hydrogen Carbonates
using heat
forms a metal carbonate &
evolves a CO2 gas & H2O
(can be tested with the limewater test, and
if H2O is produced it will condence on test tube)
metal hydrogen carbonate (heat) = metal carbonate + carbon dioxide + water
Catalytic decomposition:
Decomposition reactions speed up
by the addition of a Catalyst
Some substances can undergo decomposition without heating, they
slowly undergo decomposition all on their own
. However the reaction maybe
sped up by the addition of another substance, called a Catalyst
Catalyst = A substance that increases the rate of reaction of a chemical reaction,
without being used up in the reaction
How a Catalyst works:
1) Provide
alternate reaction pathways
, witch lower activation energy (now its easier)
2) Therefor,
energy requirement
for a successful collision resulting gin a reaction,
decreases
3) The
successful collisions rate
of particles,
increases
4) the
Reaction Rate is faster
5) Products are formed more rapidly (e.g. bubble will form at the faster rate)
example of excellence answer:
When a catalyst is added to a solution it creates alternate reaction pathways so the activation energy required for the reaction decreases. this means the collisions of reactant particles don;t need as much energy to be successful. there is a greater number of successful collisions that form products, so the reaction rate increases. while the catalyst is involved with the reaction, by reducing the energy barrier, the substance is not used up in the reaction, and will remain after it is finished
Hydrogen Peroxide
, decomposition reaction with a catalyst
Hydrogen Peroxide = H2O2(aq)
Hydrogen Peroxide is a substance that undergoes
decomposition slowly at room temperature
water (H2O) & Oxygen gas (O2) are formed in this reaction
(bubbles of oxygen gas are observed as the decomposition occurs + a glowing splint will re-light )
there are
2 catalysts that can assist with the reaction:
manganese Dioxide:
black powder stays black
fizzes rapidly
test tube heats up (exothermic)
Potassium Iodide:
white powder turns yellow/orange
fizzes rapidly
test tube heats up (exothermic)